: The Oxford English Dictionary says "Origin unascertained". The earliest use it gives is from Munchhausen's Travels : "In six jiffies I found myself and all my retinue . . . at the rock of Gibralter [sic]". I'm told that the term was taken into the scientific community late in the 20th century and was actually applied as a unit of time; however the unit seems to vary from discipline to discipline!

"jiffy, adj. 1785, a very short space of time; of unknown origins." (From "The Barnhart
Concise Dictionary of Etymology: the Origins of American English Words" by Robert K. Barnhart, HarperCollins Publishers, New York, 1988.)